The present invention relates to a new and improved method and apparatus for regulating the melting rate of a self-consuming electrode in a slag bath during electroslag remelting.
In Austrian Pat. No. 345,487 it has already been proposed to keep constant the current required for remelting in that, upon deviations from a reference or set value there is altered the lowering or immersion speed of the electrode which is to be melted. This electrode lowering speed is increased when the current is too low and reduced when the current is too high. Moreover, when the current is maintained constant in this manner there is increased the bath potential for the purpose of inputting a greater amount of actual or nonreactive power when the lowering speed of the electrode is too low with respect to a predetermined lowering speed. This enables increasing the melting rate, and thus, the lowering or immersion speed of the electrode. Conversely, if this lowering speed is too high the bath potential is reduced.
However, when practicing this method there is not afforded the possibility of intentionally proportionally influencing the melting rate, because with the adjustment of the bath potential or voltage, there is, in turn, altered the lowering or immersion speed of the electrode. For instance, when the current is regulated to be constant a voltage increase would cause a reduction in the immersion depth of the electrode into the molten bath, whereby, however, the melting rate generally would not proportionally increase. Due to metallurgical reactions in the molten bath the bath resistance is altered during the course of the melting time. If the bath potential and current are maintained constant the electrode will immerse to a greater extent into the molten bath if, for instance, the bath resistance increases. Thus, the melting rate is again altered.
The aforedescribed effects do not allow for any constant remaining remelting conditions. However, for metallurgical reasons it is of great importance to establish a melting rate which is controllable, for instance a constant melting rate, because the parameters influencing the melting rate and also the immersion depth of the electrode into the molten bath are multifarious and cannot be precisely calculated if melting rate and immersion depth are to be supervised by regulation techniques or methods.
According to German Pat. No. 1,934,218, for instance, the melting weight of a self-consuming electrode is regulated according to a weight-time function without taking into account an electrode spacing which is to be maintained. In German Pat. No. 2,456,512 the immersion depth is regulated according to the bath resistance or its gradients, without monitoring the melting rate. The aforementioned reasons make it readily apparent that regulation of the immersion depth as a function of the bath resistance is very inexact.
The heretofore known methods for regulating the melting rate are afflicted with the disadvantage that such regulation only is performed according to the advance or feed speed of the electrode, the voltage and the current intensity. However, the position of the electrode in the slag bath and its distance from the melt level or meniscus are not taken into consideration. Yet, for the metallurical characteristics of the ingot or block to be molten the thermal conditions during solidification are extremely important. The deeper the electrode immerses into the slag bath, the higher the temperature of the still liquid ingot or block and the deeper the sump of liquid metal formed in the ingot or block. Furthermore, the melting rate is not a linear function of the immersion depth, which makes the immersion depth an essential control parameter or magnitude.